Patent GB-1154598-A (GB)
Title: Improvements in and relating to Diamond Grit Materials.
IPC: B24D18/00
Filed: 1966-08-31
Abstract: 1,154,598. Abrasive tools. SCANDIAMANT A.B. 31 Aug., 1966 [22 Sept., 1965; 19 Oct., 1965; 2 Feb., 1966; 29 March, 1966], No. 38820/66. Heading B3D. [Also in Divisions C4 and C7] Coated diamond grits when used in the manufacture of grinding material, in which the diamond grits are embedded in a resin bond are characterized in that the grits are individually coated with a layer of metallic material said layer having a thickness of 0.5 to 25 microns. The preferred range of thickness of the metallic layer is from 10 to 15 microns and the layer comprises as the main constituent, one or more of the metals, nickel, cobalt, silver, copper, molybdenum, titanium, aluminium, manganese, cadmium, tin, zinc, chromium, tungsten, iron or zirconium, or an alloy or mixture containing one of these metals. The diamond grits can each be coated with the layer by chemical reduction, electrolysis or by vaporizing the metal on to the diamond grit. In one example of chemical reduction, nickel is reduced on to diamond grits of 140 to 170 mesh size in an autoclave under hydrogen and a pressure of 20 atmospheres from an aqueous solution of a nickel salt containing 230 grams per litre NiSO 4 .6H 2 O, 300 grams per litre (NHu) 2 SO 4  and 130 millimetres concentrated aqueous ammonia per litre, the diamond grits being dispersed in the solution. The autoclave is equipped with a stirrer and the reduction takes place at a temperature of from 160‹ to 170‹ C. the reduction being started by adding small amounts of a catalyst such as anthraquinone and is continued until the desired thickness of the metallic layer on each grit has been reached; an example is also given in which a cobalt-nickel layer is precipitated on each grit. As an example of coating by using electrolysis, synthetic diamond grits of 80 to 100 mesh are cleaned by boiling in concentrated HNO 3  and rinsed in distilled water, the diamond grits are then silvered by reduction of silver from an ammoniacal solution of silver nitrate and a reducing agent to obtain a silver layer on each grit which is less than 0À25 micron in thickness; the silvering makes the diamond grits electrically conductive. Seventy - five carats of the silvered diamond grits are transferred to a rotatable cylindrical drum containing an aqueous solution of NiSO 4 .6H 2 O (140 grams per litre), NH 4 Cl (30 grams per litre), H 3 BO 3  (30 grams per litre), a nickel anode is then placed above the silvered diamond grits while the drum containing the grits serves as the cathode; the current is switched on and the current density is adjusted to a value of about 20 ma/cm.<SP>2</SP> area of the top layer of the grits facing the anode. The drum is rotated at a speed of 1 to 2 r.p.m. The electrolysis is continued for about 24 hours whereupon the now coated grits are removed, rinsed and dried. The thickness of the metallic layer of nickel on each is by then about 25 microns. As an alternative to silver the conducting layer can be gold, platinum group metals such as platinum or palladium, copper, nickel, aluminium or cobalt. To produce the grinding material the coated diamond grits are mixed with a resin containing a filler and the mixture is cold pressed and then heated to a suitable curing temperature in a mould until the resin is cured; a suitable resin bond is prepared from a phenolic resin of the novolak type (1À2 moles phenol per mole formaldehyde) containing sixteen parts by weight hexamethylene tetramine and 100 parts by weight of novolak which is heated to 110‹ C. whereupon it is crushed and mixed with a filler consisting of silicon carbide and calcium oxide (resin 38À8% by weight, calcium oxide 1À2% by weight, silicon carbide (600 mesh) 60% by weight Other resinous binders referred to are epoxy resins, polyimides, alkyd resins, unsaturated polyester resins, silicones, polybenzinimidazoles and polyamidimides.